That was Eli's point -- it doesn't work with C-h -- please try it.
C-h
;; Shows first "C-h (Type ? for further options)-", then after a secondthe
;; which-key pop-up occurs.
C-h
;; Now the C-h help help buffer will be shown.
C-n
;; Now you're viewing the NEWS file.
I.e., you cannot paginate through the commands if you are issuing C-h
and have which-key-mode on.
You also can't invoke describe-prefix-bindings from C-h in vanilla emacs. The reason is the same. which-key is just intercepting the help-char to do the pagination.